luckyflyer

I'm new to jets so pardon me if this is a dumb question. When I defuel my Eurosport I see air bubbles in the line to my 5 gal jug. I have no leaks or drips. All of the lines are tight. I can't figure out where this air is coming from, any Ideas? Or is there no need to be concerned about this.
Luckyflyer

rcpete347

My Feedback: (251)

HI, you should have no bubbles, defueling is a good way to check for air leaks. check the UAT connections, loose the fuel line nut on the UAT and tie wire every thing.
Rcpete

Xwingnut

My Feedback: (24)

I notice the same thing. Although, only on defueling. I just figured the field pump is so strong it makes the fuel cavitate in the lines. I have vacuumed checked the fuel system and there are no leaks anywhere to be found.

Xwing

C185Pilot

My Feedback: (25)

Mine cavitates and creats bubbles...no leaks.

Mike

dubd

I get bubbles when defueling. I've noticed this with others as well.

jeteffx

My Feedback: (2)

sometimes you will get bubbles if you defuel too fast, the pumps on our gas containers flow a much higher rate that the fuel pump on our engines, try using a 4.8 cell pack to power your defueling, you might be surprised to not see any bubbles then........this is what I have done in the past....Perry

luckyflyer

Thanks all, I'm going to quit worrying about it.
Luckyflyer

BaldEagel

Most people with UAT's fuel and de-fuel from the fitting on the top edge, this fitting is not submersed in fuel and has to suck out the air already in the UAT, once it has done that it then has to lift the fuel through thin air, reliant on the pump suction, I think this is where most of the air when defueling comes from, I came to the conclusion that filling and de-fueling on the wrong side of the main tank clunk/filter was not in my turbines best interest as I was bypassing a filter, so I put a seperate short connection and filter clunk in the main tank to fuel and de-fuel from.

Mike

hooker53

My Feedback: (106)

I never pull mine out. It's low on fuel when I land anyway, so I just put a small Length of tubing with a plug, on the vent and load it up.

SinCityJets

Getting bubbles while de-fueling is common. Cavitation is the cause. Generally, I do not de-fuel my plane if I flew my standard flight time.

Chad

hooker53

My Feedback: (106)

[sm=thumbup.gif]

tassos p

The reason is simple. When pressurizing ie when fuelling or when the engine pump is operating the system is pushing fuel and your system has positive pressure (means a little higher pressure than atmoshperic pressure) from the pump onwards. Before the pump however or when you defuel you have negative pressure. This means that the atmoshperic pressure is slightly higher than the pressure in your fuel lines or your defueling lines.

Most prone to this effect are the festo fittings that are designed to operate under positive pressure not negative.

You should NOT get any bubbles when defuelling. You are having a leak somewhere in your system that will never show it with fuel since it always sucks air. Of course because it is during defueling it may not worry you but you should be careful. I spent hours and hours checking my new planes and their plumming cause this is the life of my aircraft.

Tassos

FILE IFR

Good points.

I bypassed one of the cells in my Jersey Modeler fuel pump last year just for that reason... I run it at 6 volts now.

I feel it puts pressure on the tanks/UAT/Festos when fueling and de-fueling with a fast flow.

rcpete347

My Feedback: (251)

I always defuel my jet after a days flying, I always use a hand pump to defuel. If I ever see fuel bubbles, their is a leak. This is one way to stay ahead of maintenence.
Rcpete

SinCityJets

Cheap Sales Pitch to Follow:

The Orbit evoJet fuel stations (I sell) actually have a built-in speed control for forward and reverse pumping.

Chad

tothemax

I also get bubbles (when defueling) but no troubles[8D]

gooseF22

Another way to defuel:

just hook your fuel can up to the jet and elevate the jet above the fuel can. it will siphon while you put your other stuff away. I fueled the other day and forgot to disconnect, and it emptied the tank in about 15 min..

invertmast

My Feedback: (23)

+1 but if the jet is staying in the trailer for prolonged periods of time, i pull the vent plug off so the tanks don't split (yea i had a stupid moment once)[:@]

hooker53

My Feedback: (106)

oooH Thomas, I'v turned my Sq. tank into a round one more than once!!! ha. I wondered if anyone else has done that!!! ha. Lucky enough, I caught it before it was too late. Sure told me a lot about my sealing process!!! I have a little placecard with the words "remove vent plug before fueling" beside the hatch latches Now!!! ha.

u4eake

Kerosine can contain up to 10% of dissolved air. When pressure drops suddenly (like when sucking fuel out hard) this air will come out of the fuel, just like CO2 out of a can of soda.

Take a veterinary serynge and fill it halfway with kero. Block the intake and suck it vaccuum. Watch the kero foam and sparkle.

It's also similar to caisson's desease with divers : under high water pressure, nitrogen dissolves in the blood. When surfacing, pressure drops quickly and the nitrogen comes out of the blood, making it foam in your vains.<br type="_moz" />

Craig B.

.......correct and this, together with the high flow rate generated by the pump generates a low pressure in the fuel line which causes cavitation. This is NOT indicative of a leak in your fuel system! It is indicative of how easily kerosene vapourizes. Pour it on your garage floor and watch it evaporate!

Wrong! Festo fittings are designed to operate under positive pressure but are also rated to approximately negative one atmosphere.

Wrong again. Refer to my first comments. This is quite common and not necessarily indicative of a problem, but it can appear that way.

Craig.

Carsten Groen

I took a fueltank, with just two connections (glued in, NO leaks), I sucked the fuel (thru a "JetCat" felt clunk) out of it with a hose connected to one of the connections, the other connections was open.
I got bubbles in the fuel (when the pump was running fast) !
(Also had the same problem in my Viperjet, above a certain setting of the fuelpump, there would be small "micro" bubbles in the fuel, this was also not because of leaks, but only because of the high flow/cavitation)
So bubbles do occur, it's not a proof of a leak somewhere (although a leak will also show itself like this...)